It is well known in the petroleum field that petroleum which is found in subterranean reservoirs is recovered by many different methods. The primary method of petroleum recovery is by the primary recovery means which employs natural forces such as pressure, either by the petroleum itself or by the presence of gases, whereby petroleum is forced from the subterranean reservoir to the surface and recovered. Subsequent to the recovery of the petroleum by the primary means, due to the dissipation of the natural or gaseous pressure, more of the petroleum in the reservoir may be recovered by a secondary process in which water is forced into the reservoir to provide the pressure necessary to force the petroleum from the reservoir to the surface.
At some point in the recovery of petroleum, a state is reached in which it is more costly to use the water pumped in relative to the amount of oil which is recovered by this method. However, inasmuch as a relatively large amount of petroleum may still be present in the reservoir, either in a pool or by being trapped in interstices of relatively porous rock, it is necessary to effect the recovery of the petroleum by a tertiary method. The tertiary method or the enhanced oil recovery method may be effected by many different methods. For example, one tertiary recovery method may be thermal in nature in which steam is pumped into the reservoir to force the oil to the surface. However, some oil may be lost due to burning and, by combining the cost of the lost oil with the cost of the equipment and energy necessary to form the steam, may render such a method economically unattractive to operate. A second tertiary recovery method may comprise a fire flood method in which a portion of oil is ignited to create gases as well as reducing the viscosity of the heavy crude with a concomitant increase in pressure to force the oil from the reservoir. However, as in the method previously discussed, the drawback to this method is in the fact that some of the assets are being destroyed, thus increasing the cost of the operation. A third method for enhanced oil recovery is in the use of carbon dioxide to provide the pressure required to force the oil to the surface. In this method, the carbon dioxide is pumped into the oil reservoir to dissolve some of the heavies present which, in turn, will reduce the viscosity and allow the oil to reach the surface. The disadvantage which is present when utilizing such a method is the requirement for relatively expensive equipment to produce the carbon dioxide. In addition, the method is dependent upon the ready availability of carbon dioxide. Yet another method for enhanced oil recovery is found in the use of chemicals such as water-soluble polymers including polyacrylamide, biopolymers, etc. These polymers will increase the viscosity of the water in the solution and render the mobility ratio of water to oil more favorable whereby the solution will act more favorably as a plug.
Another method of effecting an enhanced oil recovery is by utilizing surfactants as a plug, whereby the oil which is present in the reservoir may be recovered by injecting an aqueous fluid containing a surfactant or a combination of surfactants along with other compounds into the reservoir. The use of surfactants in this system is necessary inasmuch as water alone does not displace petroleum with a relatively high degree of efficiency. This occurs due to the fact that water and oil are relatively immiscible and, in addition, the interfacial tension between water and oil is relatively high. The use of surfactants will lower or reduce the interfacial tension between the water and the oil, thus reducing the force which retains the oil which has been trapped in capillaries.
The prior art is replete with various surfactants which have been used in this tertiary system for the recovery of petroleum. One type of surfactant which has been employed in many processes involves the use of a petroleum sulfonate as disclosed in U.S. Pat. Nos. 3,508,611, 3,981,361 and 4,058,467. The petroleum sulfonate fractions which are utilized in these processes have been obtained by sulfonating at least a portion of a sulfonatable hydrocarbon such as petroleum fractions as exemplified by a crude oil. However, this crude oil feedstock contains a vast and varied number of chemical structures including aromatic hydrocarbons, paraffinic hydrocarbons, olefinic hydrocarbons, to name a few. However, a disadvantage in utilizing crude oil as a feedstock is that the feedstock usually does not contain a major portion of aromatic compounds which are the effective material which is sulfonated. As will hereinafter be shown, by utilizing a feedstock of the type of the present invention, it will be possible to obtain a greater amount of sulfonated product which is therefore available to act in the capacity of lowering the interfacial tension between oil and water when used as a surfactant.
As was previously discussed, prior U.S. patents teach the use of these petroleum sulfonates as one component of a surfactant mixture which may be used in a surfactant oil recovery process. For example, U.S. Pat. No. 4,214,999 discloses a surfactant fluid for use in flooding subterranean formations which contain petroleum comprising petroleum sulfonates possessing certain average equivalent weights and a solubilizing cosurfactant such as ethoxylated alkanols, sulfates or sufonates. U.S. Pat. No. 4,013,569 also discloses a surfactant system for the recovery of petroleum utilizing a relatively water-soluble aromatic ester polysulfonate as one component in the system. Another U.S. patent, namely, U.S. Pat. No. 4,008,165, utilizes an aqueous surfactant containing fluid which includes a mixture of three surfactant materials including a sulfonate surfactant derived from an alkyl or alkyl aromatic radical along with a phosphate ester surfactant and a sulfonated betaine, the system also being utilized in an oil recovery process.